Methods and system for controlling engine compression braking in a vehicle with a continuously variable transmission

Abstract

Methods and systems are provided for operating a continuously variable transmission (CVT) of a driveline of a vehicle during a request for engine compression braking. In one example, a method may include, responsive to an engine compression braking request, operating a CVT via a controller to adjust engine speed according to an engine speed-to-vehicle speed profile selected based on a power-based target engine speed for the engine compression braking request. The power-based target engine speed may vary continuously with driver demanded power and a battery charge power limit of the vehicle.

The invention claimed is: 
     
       1. A method, comprising:
 in response to a request for engine compression braking of a vehicle, operating a continuously variable transition (CVT) via a controller to adjust engine speed according to an engine speed-to-vehicle speed profile selected based on a power-based target engine speed for the compression braking request, the power-based target engine speed varying continuously with driver demanded deceleration power and a battery charge power limit of the vehicle. 
 
     
     
       2. The method of  claim 1 , wherein operating the CVT via the controller to adjust engine speed according to the selected engine speed-to-vehicle speed profile includes commanding engine speed to a level determined from the selected engine speed-to-vehicle speed profile as a function of current vehicle speed. 
     
     
       3. The method of  claim 1 , wherein the driver demanded deceleration power includes a negative wheel power demand and the power-based target engine speed is determined based on the negative wheel power demand during the request for engine compression braking and the battery charge power limit of a battery of the vehicle, where the vehicle is a hybrid vehicle. 
     
     
       4. The method of  claim 1 , wherein the engine speed-to-vehicle speed profile is a first engine speed-to-vehicle speed profile selected from a plurality of engine speed-to-vehicle speed profiles stored in a memory of the controller. 
     
     
       5. The method of  claim 4 , wherein the first engine speed-to-vehicle speed profile is a lowest engine speed-to-vehicle speed profile out of the plurality of engine speed-to-vehicle speed profiles and further comprising selecting the first engine speed-to-vehicle speed profile at a start of engine compression braking, after receiving the request for engine compression braking. 
     
     
       6. The method of  claim 5 , further comprising changing the selected engine speed-to-vehicle speed profile from the first engine speed-to-vehicle speed profile to a higher, second engine speed-to-vehicle speed profile of the plurality of engine speed-to-vehicle speed profiles in response to the power-based target engine speed increasing above an upper threshold, the upper threshold determined based on a percentage of a difference between a second engine speed specified according to the second engine speed-to-vehicle speed profile for a current vehicle speed and a first engine speed specified according to the first engine speed-to-vehicle speed profile for the current vehicle speed. 
     
     
       7. The method of  claim 6 , further comprising increasing engine speed from the first engine speed to the second engine speed in response to changing the selected engine speed-to-vehicle speed profile and continuing to adjust engine speed to a level specified by the second engine speed-to-vehicle speed profile as the current vehicle speed changes. 
     
     
       8. The method of  claim 4 , further comprising selecting the first engine speed-to-vehicle speed profile, based on the power-based target engine speed, in response to an engine speed specified by the first engine speed-to-vehicle speed profile for a current vehicle speed most closely matching the power-based target engine speed relative to engine speeds specified by remaining engine speed-to-vehicle speed profiles of the plurality of engine speed-to-vehicle speed profiles. 
     
     
       9. The method of  claim 8 , further comprising changing the selected engine speed-to-vehicle speed profile from the first engine speed-to-vehicle speed profile to a second engine speed-to-vehicle speed profile of the plurality of engine speed-to-vehicle speed profiles in response to the power-based target engine speed increasing above an upper threshold or decreasing below a lower threshold, the upper threshold and the lower threshold determined based on a difference between a second engine speed specified according to the second engine speed-to-vehicle speed profile for a current vehicle speed and a first engine speed specified according to the first engine speed-to-vehicle speed profile for the current vehicle speed, the upper threshold and lower threshold being different and both at levels between the first engine speed and second engine speed. 
     
     
       10. The method of  claim 1 , wherein the request for engine compression braking is generated in response to a battery of the vehicle not being able to accept power for producing a negative wheel torque to meet the driver demanded deceleration power for a deceleration event. 
     
     
       11. A method, comprising:
 during a vehicle deceleration event where requested deceleration torque cannot be delivered via negative motor torque:
 operating a continuously variable transmission (CVT) via a controller to adjust engine speed according to a first engine speed-to-vehicle speed profile in response to a request for engine compression braking; and 
 changing operating the CVT via the controller to adjust engine speed according to a second engine speed-to-vehicle speed profile in response to a change in wheel torque request that increases or decreases a power-based target engine speed above or below respective thresholds. 
 
 
     
     
       12. The method of  claim 11 , wherein the power-based target engine speed is a target engine speed for delivering a compression braking power request, the compression braking power request based on the wheel torque request and a battery charge power limit of a battery of a hybrid vehicle. 
     
     
       13. The method of  claim 11 , wherein operating the CVT via the controller to adjust engine speed according to the first engine speed-to-vehicle speed profile includes adjusting engine speed to a level determined from the first engine speed-to-vehicle speed profile for a current vehicle speed and continuing to adjust the engine speed according to the first engine speed-to-vehicle speed profile as vehicle speed changes. 
     
     
       14. The method of  claim 11 , wherein the respective thresholds are determined based on a percentage of an absolute value of a difference between a first engine speed specified by the first engine speed-to-vehicle speed profile at a current vehicle speed and a second engine speed specified by the second engine speed-to-vehicle speed profile at the current vehicle speed. 
     
     
       15. The method of  claim 14 , wherein changing operating the CVT includes changing operating the CVT via the controller to the adjust engine speed to the second engine speed in response to the power-based target engine speed increasing above an upper threshold, the second engine speed being higher than the first engine speed and the upper threshold being closer to the second engine speed than the first engine speed. 
     
     
       16. The method of  claim 14 , wherein changing operating the CVT includes changing operating the CVT via the controller to the adjust engine speed to the second engine speed in response to the power-based target engine speed decreasing below a lower threshold, the first engine speed being higher than the second engine speed and the lower threshold being closer to the second engine speed than the first engine speed. 
     
     
       17. A system for a hybrid vehicle, comprising:
 a continuously variable transmission (CVT); 
 a battery coupled to a motor of the hybrid vehicle; 
 a controller with computer readable instructions stored on non-transitory memory that when executed during a deceleration event of the hybrid vehicle, cause the controller to:
 operate the CVT to adjust engine speed to a first level determined based on a current vehicle speed and a first engine compression braking profile of engine speed vs. vehicle speed; 
 determine a power-based target engine speed to deliver a compression braking power request based on a difference between a driver demanded deceleration power for the compression braking power request and a battery charge power limit of the battery; and 
 in response to the determined power-based target engine speed increasing above a first threshold based on engine speeds specified by the first engine compression braking profile and a higher, second engine compression braking profile, shift to the second engine compression braking profile and adjust engine speed to a second level determined based on the current vehicle speed and the second engine compression braking profile. 
 
 
     
     
       18. The system of  claim 17 , wherein the instructions further cause the controller to: in response to the determined power-based target engine speed decreasing below a second threshold based on engine speeds specified by the first engine compression braking profile and a lower, third engine compression braking profile, shift to the third engine compression braking profile and adjust engine speed to a third level determined based on the current vehicle speed and the third engine compression braking profile. 
     
     
       19. The system of  claim 17 , wherein the instructions further cause the controller to: in response to the determined power-based target engine speed increasing above a fourth threshold based on engine speeds specified by the second engine compression braking profile and a higher, further engine compression braking profile, shift to the fourth engine compression braking profile and adjust engine speed to a fourth level based on the current vehicle speed and the fourth engine compression braking profile. 
     
     
       20. The system of  claim 17 , wherein operating the CVT to adjust engine speed to the first level and adjusting engine speed to the second level is responsive to and occurs during a compression braking condition where a requested deceleration torque cannot be delivered via negative motor torque of the motor.